The third generation partnership project (3GPP) is developing a Long Term Evolution (LTE) specification for the purpose of facilitating deployment of broadband services and applications wirelessly.
LTE is designed for uplink speeds up to 50 Mbps and downlink speeds of up to 100 Mbps for high speed data and media transport. Bandwidth will be scaleable from 1.25 MHz to 20 MHz. This will provide different network operators the ability to have different bandwidth allocations and provide different services based on spectrum. The provision of such an arrangement of scaleable bandwidth is expected to allow carriers to provide increased data and voice services over a given bandwidth, since bandwidth can be more properly matched to the needs of a given application than has heretofore been possible.
LTE employs advanced technologies that are relatively new to wireless cellular networks, including orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO) antenna technologies. The uplink utilizes single carrier frequency division multiple access (SC-FDMA) while the downlink employs orthogonal frequency division multiple access (OFDMA). The basic operation and technical description of LTE may be found in the current draft of the 3GPP LTE specification, 3GPP (or 3G) Release 8, including 3GPP TX 36.XXX V8.3.0 (2008 May), which are incorporated herein by reference. The operation and technical description of UMTS may be found in the current draft of the UMTS specification, SGPP TS 25.XXX, which is incorporated herein by reference.
LTE transmissions are segmented into frames of 10 mSec duration. Each frame is divided into 10 sub-frames each having two slots. Within each slot, a number of OFDM symbols (6 or 7) are transmitted. The transmitted downlink signal includes N subcarriers (15 KHz) for a duration of M OFDM symbols (typically 6 or 7) which may be represented by a resource grid. FIG. 1 shows an exemplary resource grid illustrating a sub-frame with two slots (time; x dimension) and twelve subcarriers (frequency; y dimension). Each slot includes seven OFDM symbols (0 thru 6). As will be appreciated, the total number of subcarriers will depend on the overall transmission bandwidth.
Each block within the grid is referred to as a resource element (RE). Reference signals/symbols (R) (not shown in FIG. 1) are transmitted during certain OFDM symbols of each slot and transmitted every sixth subcarrier (resulting in staggered Rs in both time and frequency).
In OFDMA, users are allocated a specific number of subcarriers (frequency) for a predetermined amount of time. These are referred to as physical resource blocks (PRBs) which are allocated using a scheduling function. A PRB is defined as 12 consecutive subcarriers for one slot.
Within an LTE communication system, base stations may utilize one of a number of available antenna diversity schemes based on the number of transmit antenna ports for downlink transmission to the user equipment (UE). In the currently drafted standard, three antenna diversity schemes are provided which correspond to 1, 2 or 4 transmit antenna ports. These may also be referred to as “sets” of transmit antenna ports. While this configuration is exemplary, the base station may have any number (2 or greater) of transmit antenna ports and thus, may operate in one of a number of diversity schemes. The UEs include one or more antennas and transceivers enabling receipt signals transmitted according to the antenna diversity scheme (e.g., 1, 2 or 4 transmit antenna ports). Knowing the number of base station transmit antennas (antenna configuration) is critical information for the UE because it is necessary to decode data transmission correctly after initial access. For example, utilization of two or four base station transmit antenna ports, as compared to one, increases system data rates, reliability and/or quality of service.
Within the present LTE standard, the synchronization channel(s) do not carry any transmit antenna configuration information. Under the current scheme, the UE detects the number of transmit antenna ports by determining which transmit antenna diversity scheme is being deployed. Each of the three base station transmission modes (e.g., using 1, 2 or 4 transmit antenna ports) has its own antenna diversity scheme: 1 (Single Input Multiple Output, or SIMO), 2 (Spatial Frequency Block Code, or SFBC) and 4 (Spatial Frequency Block Code—Frequency Switched Transmit Diversity, or SFBC-FSTD). By detecting the appearance of reference signal (R) subcarriers corresponding to the respective transmit antenna ports, the transmit antenna configuration can be determined by the UE. However, the reliability of such blind detection method is poor.
For the two and four transmit antenna modes, the SFBC-based transmit diversity schemes may be applied to a broadcast channel (BCH) which is transmitted within a predetermined portion of each 10 mSec frame (e.g., as a portion of the frame). As currently proposed in the LTE specification, the BCH is transmitted in the first sub-frame (sub-frame 0) of each frame and included within OFDM symbols 0 thru 3 in slot 2. The primary and secondary synchronization signals are transmitted in the first and sixth sub-frames (sub-frame 0 and sub-frame 5) and included within OFDM symbols 5 and 6 within slot 1 of these sub-frames. This is illustrated in FIG. 2 which shows an example resource grid for sub-frame 0 containing the BCH and synchronization channels when transmitting using 2 transmit antenna ports.
The reference signals are denoted “R1” for transmit antenna port #1 and “R2” for transmit antenna port #2. As is understood, the resource elements identified as R2 are unused in antenna port #1 transmissions and those identified as R1 are unused in antenna port #2 transmissions. It will be appreciated that in this diversity scheme (using 2 transmit antenna ports), the resource elements for transmit antenna ports #3 and #4 within the BCH are unused and denoted with an “X”.
The data transmitted in the BCH contains vital system and access configuration information the UE requires in order to access the system, such as system bandwidth, system frame number, basic configuration required for further decoding of other information/data, and configuration information for various operational features. This channel typically utilizes a low coding rate as well as 16-bit cyclic redundancy check (CRC). This system and access configuration information may be referred to as the BCH transport block data. Thus, the data transmitted in the BCH includes two distinct segments: the transport block data and CRC parity bits (computed from the transport block data). It was expected that reception of the BCH would allow the UEs to determine the number of transmit antenna ports in the base station by recognizing the antenna diversity scheme.
However, under the proposed scheme, the number of transmit antenna ports cannot be adequately detected solely on the basis of the different transmit diversity schemes. This is because each transmission scheme has a large portion of its signal which is identical for all the transmit antenna diversity schemes. Below is a representation of the three proposed schemes (SIMO (1 antenna port), SFBC (2 antenna ports) and SFBC-FSTD (4 antenna ports)):
      1    ⁢                  ⁢    antenna    ⁢                  ⁢          (      SIMO      )        ⁢          :        ⁢                  ⁢          ⌊                                                  S              1                                                          S              2                                                          S              3                                                          S              4                                          ⌋            2    ⁢                  ⁢    antennas    ⁢                  ⁢          (      SFBC      )        ⁢                  :            ⁢                          [                                                  S              1                                                          S              2                                                          S              3                                                          S              4                                                                          -                              S                2                *                                                                        S              1              *                                                          -                              S                4                *                                                                        S              3              *                                          ]            4    ⁢                  ⁢    antennas    ⁢                  ⁢          (              SFBC        ⁢                  -                ⁢        FSTD            )        ⁢                  :            ⁢                          [                                                  S              1                                                          S              2                                            0                                0                                                0                                0                                              S              3                                                          S              4                                                                          -                              S                2                *                                                                        S              1              *                                            0                                0                                                0                                0                                              -                              S                4                *                                                                        S              3              *                                          ]      The columns represent different neighboring subcarriers while the rows represent transmission from different transmit antenna ports. Between SIMO and the SFBC transmission fully half of the transmission (e.g., transmit antenna port #1) is identical. This is also true to a lesser extent between SIMO and SFBC-FSTD where the first two signals (S1 and S2 for transmit antenna port #1) are identical. Because the coding rates of the BCH is extremely low (approximately 1/14), UEs with even moderately good channels will be able to correctly decode the BCH using the incorrect number of transmit antenna ports (or diversity scheme). In operation, the UEs decode the BCH using each of the three possible schemes, perform CRC operation on the decoded data, and compare it to the received CRC. It is possible that a UE may correctly decode the BCH using the incorrect number of transmit antenna ports. Therefore, the UE may determine that the base station is transmitting using one scheme (1, 2 or 4 transmit antenna ports), when in fact, it is transmitting using a different scheme. Additionally, any method which is based on the relative structure of these signals would fail when either one of the antennas channels is in deep fade, or two of the antennas channels are very similar to each other.
Accordingly, there is needed a more robust and reliable method of detecting the number of base station transmit antennas to improve antenna configuration detection performance.